Add Ntfy push notifications for super admin events

Sends alerts to a configurable Ntfy topic on: new user registration,
account lockout after 5 failed login attempts, and login attempts on an
already-locked account. Fire-and-forget — never raises if Ntfy is down.

Configure via NTFY_URL and NTFY_TOKEN in .env.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/config.py

@@ -12,6 +12,8 @@ class Settings(BaseSettings):
     admin_username: str = "admin"
     admin_password: str  # no default — must be explicitly set in .env
     docs_enabled: bool = False
+    ntfy_url: str = ""       # e.g. https://ntfy.sh/your-secret-topic
+    ntfy_token: str = ""     # optional Bearer token if topic is protected
 
     @property
     def cors_origins_list(self) -> list[str]: